Control circuits have previously been utilized for controlling the rate of feed of material to a utilization device. As one example, coal on a conveyor belt may be fed at a variable rate by variable speed of the motor driving the conveyor belt and the actual coal per-unit-length of conveyor belt may vary according to the amount of coal dropping out of a bunker or chute onto the conveyor belt. Accordingly, the rate of feed is the multiplication product of the weight-per-unit of belt length times the speed of the conveyor belt.
The weight signal may be generated by a transducer, for example, a load cell, which converts the force or weight of material into an electrical signal. Belt travel may be obtained by an odometer or tachometer that generates a pulse per unit of belt travel or generates a frequency proportional to belt speed. A prior art system for performing this multiplying product is to transmit the load cell signal and odometer signal to a distant electrical cabinet whereat the load cell signal is amplified, converted into a digital signal, and then multiplied by the belt speed signal. This prior art system has at least three disadvantages:
(1) It requires the transmission of the load cell output signal, which is a low level signal of generally a few millivolts, over a long conductor. For this reason, the load cell wiring requires special precautions to eliminate the noise induced by electromagnetic radiation. Also, errors are introduced by the thermocouple effect between wire connections.
(2) The electronics require considerable programming to scale the system to the required demand and to provide the correct feedback signal. Usually the prior art systems reverted to a scaling of both the weighing signal and the belt speed signal into a combined percentage signal in order to accommodate system variations.
(3) The circuit requires the use of an analog-to-digital converter to digitize the weighing signal. These converters are expensive and introduce errors for which compensation is extremely difficult.